chore(logger): serialize DateTime to ISO strings

[jamietanna]

Changes

As an alternative to 2228a79, we can
instead serialize any DateTime objects that are present in our logging
fields.

Context

Please select one of the following:

• This closes an existing Issue, Closes: #
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

AI assistance disclosure

Did you use AI tools to create any part of this pull request?

Please select one option and, if yes, briefly describe how AI was used (e.g., code, tests, docs) and which tool(s) you used.

• No — I did not use AI for this contribution.
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• Yes — substantive assistance (AI-generated non‑trivial portions of code, tests, or documentation).
• Yes — other (please describe):

Documentation (please check one with an [x])

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• No documentation update is required

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I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests, but ran on a real repository, or
• Both unit tests + ran on a real repository

The public repository:

DEBUG: latest repository commit (repository=JamieTanna-Mend-testing/renovate-iss-41260-gomod-bug-test)
       "latestCommit": {
         "hash": "fc47f18e463d1ce2e811e73b4f3e1c188bdbb3fe",
         "date": "2026-02-17T18:54:05+01:00",
         "message": "Update main.go",
         "refs": "HEAD -> master, origin/master, origin/HEAD",
         "body": "",
         "author_name": "Michael Walsh",
         "author_email": "59025052+walsm232@users.noreply.github.com"
       }